Thin film semiconductor device having a gate electrode insulator formed through high-heat oxidization

ABSTRACT

A thin film semiconductor device includes a gate electrode insulator formed through high-heat oxidization of a semiconductor film. The high-heat oxidization of semiconductor film is carried out, in the process of crystallization or recrystallization of non-single-crystalline semiconductor thin film on a base layer, by irradiating predetermined areas of the thin film which is implanted with oxygen ion before irradiation, to convert such areas to oxidized areas, and these areas are processed to gate electrode insulators of electric circuit units in the thin film semiconductor device.

[0001] This application claims the benefit of the filing date ofJapanese patent application serial number 2001-317414 entitled “A ThinFilm Semiconductor Device Having A Gate Electrode Insulator FormedThrough High-Heat Oxidization” which was filed on Sep. 10, 2001.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a thin film semiconductor deviceand a semiconductor substrate sheet to be used in the semiconductordevice as well as a method for producing them.

[0003] As is well known, a thin film semiconductor device (also known asa thin film transistor (TFT) device) is formed on a semiconductorsubstrate, which typically consist of a thin film layer of semiconductormaterial, such as silicon, over a base layer of insulation material,such as non-alkaline glass, or quarts glass. In the thin film layer ofthe semiconductor, a plurality of channels consisting of a source areaand a drain area are formed, and each of channels is equipped with agate electrode separated by an insulating film from the above areas.

[0004] In a typical thin film semiconductor device, a gate electrodeinsulator is interposed between the gate electrode and the channel area.This insulator is usually formed with a film of silicon oxide, and thisfilm is typically required to be formed at a low temperature. To formthis silicon oxide film in a TFT, high temperature silicon oxide filmformation techniques such as those used in large scale integration (LSI)semiconductor processes (which may require temperatures of more than900° C.) typically cannot be used. Instead, a relatively low temperaturedeposition process (e.g., a temperatures less than 600° C.), such as oneusing a plasma CVD method, is used.

[0005] Although an oxidized film deposited by the plasma CVD method canbe used to form an insulator film for a TFT, it may have disadvantagesin insulation property and/or stability compared with a film oxidized ata high temperature. This occurs because, when using the plasma CVDmethod, some impurities remain between the channel area and the gateinsulator film, and further, the resultant silicon oxide film has atendency to be composed of compounds as which do not have astoichiometrically regulated composition of “SiO₂” but, instead, have anirregular composition such as “SiO_(1.9)”. When an oxidized film havingsuch characteristics is used as a gate electrode insulator of a TFT, theTFT circuit tends not only to have greater variance of threshold voltagevalues, but also have reduced long-term stability of TFT properties. Forexample, in conventional products, variation of TFT threshold voltagevalues may be in the ±0.4V range and the magnitude of this variation mayincrease over time.

[0006] Furthermore, in conventional thin film semiconductor devicesusing poly-crystalline silicon, disadvantages due to the small size ofcrystal grains and the irregularity of configuration mode of crystalgrains are inevitable. That is, as a poly-silicon film is composed of anumerous crystal grains of extreme small size, the improvement ofmobility is limited due to such phenomenon as dispersion of electrons orholes at boundaries between crystal grains at the time of operation ofthe device.

[0007] Attempts have been made to use relatively large grain sizepolycrystalline silicon in order to avoid or minimize disadvantages suchas electron dispersion while simultaneously maintaining high electronmobility. For example, thin film layers having semiconductor grains ofabout 1 μm size and having a mobility of about 100 cm²/V sec. have beenformed by annealing a layer of polycrystalline silicon in a hightemperature furnace. However, high temperature annealing (e.g., over1000° C.) is used in this process, and this requires the use ofexpensive quartz glass sheets instead of relatively inexpensive sodiumglass sheets. A substrate using such expensive materials is not suitedfor cost-effective production of a device using a large substrate, suchas a TFT LCD display screens.

[0008] Other processes to obtain a thin layer film of polycrystallinesemiconductor having large size grains have been proposed. These methodsinclude irradiating a thin film of amorphous or polycrystallinesemiconductor with an energy beam (such as an excimer laser) instead ofusing high temperature annealing. By this irradiation method, it ispossible to enlarge the size of a crystal grain using relativelyinexpensive glass sheets as the base layer. However, even excimer laserirradiation is used, the size of obtained crystal grain generally doesnot exceed 1 μm. Furthermore, this excimer laser process can causeunevenness of grain size. . Incidentally, the grain size can bedetermined by “(the maximum diameter of a grain+the minimum diameter ofthe grain)÷2” and such diameters can be measured through SEM observationof crystal grains which remains after etching the film by Secco etchingprocess.

[0009] Furthermore, there is a problem on the configuration of crystalgrains in a thin film formed using the conventional excimer laserprocess. Namely, in the conventional polycrystalline semiconductor thinfilm, configuration of crystal grains in the two dimensional directionmay be highly random. The random configuration of crystal grains, andthe non-uniformity of grain size, may cause serious difficulty informing TFF devices. Such difficulties may occur because electronmobility may fluctuate when a device is formed traversing the border ofcrystal grains and, therefore, it is difficult to integrate TFT circuitshaving different channel lengths.

[0010] Accordingly, TFT devices in which polycrystalline semiconductorfilm are used may need to be designed so that each circuit extendsacross several boundaries of crystal grains in order to reduce thevariation of mobility. This is illustrated further in FIG. 7. In suchdevices, the average mobility is usually below 150 cm²/V·sec.

SUMMARY OF THE INVENTION

[0011] In general, in one aspect, implementations of the invention canprovide a thin film semiconductor device (i.e., a TFT), and a substratesheet to be used in forming the TFT device, wherein the chemicalconnection between the semiconductor film and the gate electrodeinsulator film are successive through border faces, and the gateelectrode insulator film has a stoichiochemical composition of SiO₂. Informing such a device, disadvantages of conventional TFT produced by CVDmethods can be avoided. Further, the disclosed method of deviceformation can reduce greatly the variety of threshold value and maintainstable operating characteristics.

[0012] In general, in another aspect, implementations of the inventioncan provide a thin film semiconductor device and its substrate sheet inwhich an electric circuit module can be fashioned so as to avoid theneed for arrangement of the device over many crystal grains of varioussizes and of disordered configuration, but rather, the device can bearranged to have a configuration corresponding to the arrangement ofcrystal grains.

[0013] Implementations can include features such as a thin filmsemiconductor device that can work stably for a long time with desirableoperating properties and with reduced fluctuation in operationalthreshold values. This can be provided when a partial region in thethickness direction of the thin film of semiconductor, after beingconverted to an oxidized film through irradiation of energy beam, isused as the gate electrode insulator of the device. In someimplementations, a thin film semiconductor device can have a structurein which unit circuits are arranged to correspond to the arrangement ofcrystal grains. This may be implemented when the above-describedsubstrate sheet is produced such that single-crystalline semiconductorgrains are arranged in a substantially geometric and regulararrangement.

[0014] Implementations can include a substrate sheet that comprises athin film semiconductor having a layer of semiconductor crystal grainsformed by crystallization (and/or recrystallization) of anon-single-crystalline semiconductor layer and of a layer of oxidizedfilm formed by oxidization of a non-single-crystalline semiconductorlayer. The layer of semiconductor crystal grains may have aconfiguration in which single crystalline semiconductor grains arearranged in a regular arrangement.

[0015] Implementations of the thin film semiconductor device can includea layer of semiconductor crystal grains and a layer of oxidized filmformed by oxidization of non-single-crystalline semiconductor layer, andthe oxidized film layer can be used as the insulator for the gateelectrode. Preferably, the layer of semiconductor crystal grains has acomposition in which single-crystalline semiconductor grains arearranged in a regular arrangement (i.e., an arrangement having apredictable crystalline structure).

[0016] In one implementation, the method for producing a substrate sheetfor thin film semiconductor devices includes the steps of: (a)depositing a layer of non-single-crystalline semiconductor on a baselayer of insulation materials, (b) forming oxygen implanted areas in thenon-single-crystalline semiconductor layer by implanting oxygen ionsinto the layer and (c) irradiating the layer with an energy beam. Theenergy bean changes the layer of non-single-crystalline semiconductorsuch that the oxygen implanted areas are converted to insulatingoxidized films, and other areas are converted to films of semiconductorcrystal grains. It is preferred that the irradiation by the energy beambe carried out so that areas in which the irradiation intensity aremaximum, and areas in which the irradiation intensity are minimum arearranged such that the transition of irradiation intensity between theabove two areas is successive.

[0017] Another method for producing a thin film semiconductor device caninclude steps of: (a) depositing a layer of non-single-crystallinesemiconductor on a base layer of insulation materials, (b) formingoxygen implanted areas in the layer of the non-single-crystallinesemiconductor by implanting oxygen ion into the layer, (c) irradiatingthe layer with an energy beam, thereby changing the layer ofnon-single-crystalline semiconductor such that the oxygen implantedareas are converted to insulating oxidized films and other areas areconverted to films of semiconductor crystal grains, (d) forming a gateelectrode by using the insulating oxidized films as a gate insulatorand, (e) completing an electric circuit unit by forming a sourceelectrode and a drain electrode in the layer of semiconductor crystalgrains. It is preferred that the irradiation of energy beam is carriedout so that areas at which the irradiation intensity are maximum andareas at which the irradiation intensity are minimum are arranged in aregulated mode and transition of irradiation intensity between the abovetwo areas is successive.

DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is pattern diagrams showing steps of one embodiment of theprocess according to the present invention for manufacturing a thin filmsemiconductor device.

[0019]FIG. 2 is pattern diagrams showing steps of another embodiment ofthe process according to the invention for manufacturing a thin filmsemiconductor device.

[0020]FIG. 3 is a pattern diagram for illustrating one embodiment ofdistribution of energy beam intensity in two-dimensional directions inthe irradiation step according to the process of the present invention.

[0021]FIG. 4 is a pattern diagram illustrating a profile of transitionof energy beam intensity between a maximum value and minimum value inthe process according to the present invention, being shown as asectional view along the arrow-mark in FIG. 3.

[0022]FIG. 5 is a pattern diagram illustrating an alignment state andgrowth direction of single crystalline grains during and after theirradiation of energy beam in the process according to the presentinvention.

[0023]FIG. 6 is the pattern diagrams illustrating one embodiment of thepositional relationship of electrodes with crystal grains in the thinfilm semiconductor device of the present invention.

[0024]FIG. 7 is a pattern diagram illustrating the pattern ofconfiguration of maximum intensity irradiation points and minimumintensity irradiation points such as mentioned in FIGS. 3 and 4, with athree-dimensional model pattern.

DETAILED DESCRIPTION OF THE INVENTION

[0025] To form the thin film semiconductor device of the presentinvention, it is preferred to use a glass sheet having a strain pointnot exceeding 700° C. as the material forming the base layer of asubstrate sheet. But, it is possible to use various kinds insulationmaterials other than glass, for example, ceramics or plastic filmshaving appropriate heat resistance.

[0026] On the above base layer, the single crystalline semiconductorfilm, in which an oxidized insulation film located on top of asingle-crystalline film, or in an intermediate portion of the film inthe thickness direction of the single crystalline film, is formed. Thissemiconductor thin film can be produced by implanting oxygen ions into anon-single-crystalline semiconductor film deposited on a base layer, andthen irradiating the layer with an energy beam (such as that produced byan excimer laser). This irradiation procedure can be used to change thenon-single-crystalline semiconductor film to a semiconductor filmcomposed of an oxidized film and a layer of single-crystalline grainshaving a relatively large size. In forming the non-single-crystallinesemiconductor, an amorphous semiconductor or poly-crystallinesemiconductor, in which small size crystal grains are already formed,can be used. In using the latter, the poly-crystalline semiconductor ischanged to a semiconductor film by implantation of oxygen ions andrecrystallization. The thickness of the amorphous semiconductor film ispreferred to be 30 through 300 nm, especially 30 through 200 nm.

[0027] When the non-single-crystalline semiconductor layer is formed onthe base layer, a thin control layer (a “first control layer”) foradjusting heat conduction and crystallization is formed between the baselayer and the semiconductor layer. The first control layer may be formedfrom materials such as silicon oxide or silicon nitride (SiNx). Thefirst control layer functions to block the diffusion of impurities (suchas glass components) from the base layer into the semiconductor layer,and also functions to increase uniformity of heat distribution in thesemiconductor layer. This uniformity is effected by controlling theorientation of crystallization. The thickness of the first control layercan be between 20 nm and 1000 nm, with a preferred range of 200 nm to300 nm.

[0028] A second control layer may then be formed on top of the firstcontrol layer. This second control layer has a function similar to thatof the first control layer, namely, to effect uniformity of heatdistribution and to control the orientation of crystals in thesemiconductor layer in the process of crystallization by irradiation.Materials such as silicon-oxide, silicon-nitride, silicon-ox-nitride orsilicon-carbonate (SiC) can be used for the second control layer. Thethickness of the second control layer can be between 50 nm and 500 nm,with a preferred range of 100 nm to 300 nm.

[0029] In some cases, a thin film semiconductor layer may be formedbetween the two control layers. In these cases, the material of thefirst control layer is first deposited as a thin film on the base layerof insulation material. Following this, the thin film ofnon-single-crystalline semiconductor is deposited on the first controllayer, and then the second control layer is deposited on the thin filmsemiconductor layer. Thereafter, the irradiation of energy beam from theupper direction is carried out to crystallize (or recrystallize) thelayer of non-single crystalline material.

[0030] FIGS. 1(a) through 1(e) show semiconductor layers formed atvarious process stages from deposition on a base layer to completion ofa thin film semiconductor device. In this embodiment, oxygen ions areimplanted into the top surface of non-single-crystalline semiconductorlayer.

[0031] Referring to FIG. 1(a), the first control layer 20 for heatconduction and crystallization is deposited on the glass base layer 10,and a non-single-crystalline semiconductor layer 30 is depositedthereon. Next, oxygen ions are implanted into a predetermined area toform an oxygen implanted area 33 (see FIG. 1(b)). The predetermined area33 is then irradiated with an energy beam to form an oxidized layer 40(see FIG. 1(c)). The layer 40 can be formed through high-heatoxidization of the oxygen implanted area 33 by an energy beam. Thisprocess also results in a single-crystalline semiconductor grain layer50 which is formed through single-crystallization ofnon-single-crystalline layer 30. The area which is not irradiated byenergy beam remains as a non-single-crystalline area 30. FIGS.1(c) and1(d) show the sectional view of a single area of single-crystallinegrains layer; an actual substrate sheet has a plurality of such oxidizedarea and single-crystalline grains area.

[0032] Next, the gate electrode 60 is formed on the oxidized layer 40)(which has been formed by irradiation of energy beam) using the layer 40as the gate electrode insulator (see FIG. 1(d). Then source area 70 anddrain area 71 are formed by implanting a material to form an electrode(such as phosphorous ion), into single-crystalline semiconductor layer50 using gate electrode 60 as a mask. Following this, a film 80 ofinsulation material (such as silicon oxide) is deposited so as tosurround the upper side and the lateral side of gate electrode 60 (seeFIG. 1(e)). Then, after forming contact holes in insulating film 80 atthe positions corresponding to source area 70 and drain area 71, sourceelectrode 81 and drain electrode 82 are formed by depositing materialsuch as aluminum to form an electrode; thus completing the thin filmsemiconductor device.

[0033]FIG. 2 (a) through FIG. 2(e) are pattern diagrams showing anembodiment in which oxygen ions are implanted into an intermediate layerportion of a non-single semiconductor layer.

[0034] In the FIG. 2 embodiment, the deposition of the first controllayer for heat-conduction and the non-single semiconductor layer in thestep (a) can be carried out in substantially the same way as for theembodiment of FIG. 1. However, the implantation of oxygen ions is madeinto the intermediate layer portion of the non-single-crystallinesemiconductor layer 30, and therefore, the oxygen implanted area 33 isformed in a intermediate region in the thickness direction of thenon-single semiconductor layer. After formation of oxygen implantedlayer 33, in the step(c), the layer 33 is converted to oxidized filmlayer, and non-single-crystalline semiconductor areas located at theupper-side and under-side of the oxygen implanted area are converted tosingle-crystalline semiconductor layers. Next, as shown in FIG. 2(d),the predetermined areas of the upper-side single-crystalline layer isformed as the gate electrode 60 by the patterning process. The processused in step 2(e) is substantially identical to that of step 1(e) inFIG. 1.

[0035] The volume of oxygen ion (dose) and its implanting depth (Rap)may be determined according to the thickness or position of the oxidizedinsulation layer 30. It is noted that irradiation by an energy beam isnot be limited to the use of excimer laser. For example, an argon laserradiated continuously can be used by pulsating or scanning it.

[0036] To obtain a thin film semiconductor layer in which singlecrystalline semiconductor grains are arranged in a regulated alignmentmode by irradiation of energy beams, the irradiation should be carriedout such that irradiation energy intensity changes in two-dimensionaldirections between the maximum value and the minimum value atpredetermined intervals, and maximum points and minimum points appearone after another in a regular order. In other words, the irradiationshould be carried out so that irradiated points to which maximumirradiation intensity is given and irradiated points to which minimumirradiation intensity is given are arranged in a regulated configurationsuch as a matrix-arrayed configuration.

[0037] For example, as shown in FIGS. 3 and 4, the irradiation iscarried out in a repeating energy level pattern such as the pattern“maximum value(Emax) followed by minimum value(Emin) followed by maximumvalue(Emax).” This changing pattern occurs two-dimensionally(i.e., alongboth the ‘x’ and ‘y’ axes). For example, this may be repeated across arectangle region of 5×5 mm, at every intervals of 10 μm. Further, toirradiate the entire surface, the irradiated area (for example, theabove square area of 5 mm×5 mm) may be shifted periodically in either‘x’ direction or ‘y’ direction at a predetermined pitch.

[0038] This change of irradiation energy intensity can be realized bybringing the variation to the irradiation energy intensity distributionusing a phase shift mask. Furthermore, it is desirable that the changebetween the maximum value and the minimum value be a successive changesubstantially as shown in FIG. 4.

[0039] Determination of the magnitude of the maximum value and theminimum value energy levels may be based on the film thickness of thenon-singular-crystalline semiconductor layer and the thermalconductivity of the first and the second control layers. For example,the minimum energy intensity may be determined to be an intensity atwhich the thin film semiconductor doesn't melt during the irradiation,and the maximum value may be an intensity sufficient to melt the thinfilm semiconductor during the irradiation. A melting threshold level(Emth) should be positioned between the maximum value (Emax) and theminimum value (Emin), as shown in the FIG. 4.

[0040] The face shape of the irradiation beam is not limited to a squareshape of 5×5 mm as mentioned above, and may be various polygon shapes.Further, the arrangement mode of maximum value points and minimum valuepoints is not limited to the rectangular lattice. Other shapes, such asdelta shaped lattice, also can be used.

[0041] By carrying out the irradiation of energy beam to thesemiconductor film in the manner disclosed herein, the semiconductorlayer does not completely melt in the minimum irradiation energy areas(namely, areas to which irradiation energy less than threshold value aregiven), and fine crystals of semiconductor are produced in the areasnear the threshold value areas. Some of these fine crystals functions ascores for crystallization and the crystallization progressesdimensionally from the points of these cores towards the areas to whichmaximum irradiation energy are given (i.e., in the direction of arrowmarks in FIG. 5). In the areas near where the minimum threshold valueenergy is given, at the same time as formation of the fine crystalgrains, two atoms of oxygen and one atom of silicon are bondedchemically and the formation of a SiO2 layer starts. The growth of thislayer also proceeds horizontally with the progress of crystallization ofmelted silicon.

[0042] In the areas in which the temperature of semiconductor film ishighest (corresponding to areas receiving the highest irradiation energyand, correspondingly, to the areas in which the horizontal growth ofcrystallization proceeds), a plurality of crystals which have differentor crossed crystallization growth direction collide with one another,and their contacting areas form the area of fine crystals or theborderline of crystals. Thus, when the energy beam which has a energyintensity distribution as shown in FIGS. 3 and 4 is irradiated, asubstrate for thin film semiconductor devices can be obtained in whichsingle-crystalline grains having the size of over 4 μm and covered withinsulation film surface are arranged in a regulated mode. The maximumsize of the crystal grains can be adjusted by adjusting intervalsbetween maximum irradiation energy points. Further, by varying theamount of oxygen ion to be implanted or by changing the position ofimplantation, the thickness and position (for example, at the surfacearea or inside area) of oxidized layers can be varied.

[0043] Following this process, electrode materials (such asMolybdenum-Tungsten alloy (MoW)) is deposited on the single crystallinegrain layers of the thin film semiconductor substrate. The electrodelayer forms a gate electrode and has a thickness of, e.g., 300 nm. Then,a source area and a drain area are formed using the gate electrode asthe implantation mask, followed by formation of an isolating interlayerwith insulation materials such as silicon oxide (which covers the gateelectrode). Further, contact holes are formed by perforating through thesecond control layer at the position above the source area and the drainarea, and electrode materials, such as Aluminum/Molybdenum, is depositedand patterned in the contact hole.

[0044] Thus, as shown in (a) and (b) of FIG. 6, a thin filmsemiconductor device in which one unit electric circuit is arranged ineach of a single crystal can be obtained. A thin film semiconductordevice of this type can have a high mobility (for example, over 300cm²/V·sec) exceeding the mobility of conventional devices in which asubstrate sheet comprising polycrystalline semiconductor film is used.

[0045] When an oxidized film is formed by implantation of oxygen ionsinto non-single crystalline semiconductor layer together with high-heatoxidation thereof using energy beam irradiation to form the gateinsulator, the variety of threshold voltage value (Vth) of thin filmsemiconductor device can be greatly reduced (to the degree of less than0.1 V) compared with conventional devices. Also, the stability ofthreshold value related to contamination of gate insulator or channelboundaries is greatly improved to such degree that the shift amount ofVth after 10,000 hours operation is reduced to less than 0.05 V.

[0046] These improvements appear to occur because the oxidizedinsulation film has a solid and minute character similar to that of aheat-oxidized silicon film, due to its high-heat formation processincluding a melting step. For example, the disclosed oxidized film hasproperties similar to that of high-temperature heat-oxidized siliconfilm in the value of flat band voltage obtain by leak currentmeasurement or C-V measurement, or in the shift amount of thresholdvalue in bias temperature stress estimation(BTS). Further, the disclosedoxidized film layer, not withstanding its thin character, can fullyprotect the single-crystalline silicon layer and does not contribute tobreakdown.

[0047] It is possible to omit to set the electrode on some predeterminedgrains of single-crystalline, or to set plural electric circuits to onesingle crystalline grain, if necessary. It is noted that the processdescribed above corresponds to a process for the formation of N-channelthin film transistors. This process also can be used for the formationof P channel transistors by successive implantation of impurities forthe formation of P channel transistors using partial masking means.Furthermore, it is possible to use the second control layer as anaccumulated gate insulator directly, or to use only the insulatingoxidized film as the gate insulating layer after makingetching-treatment. To reduce leakage current that may occur amongadjacent transistors, island separations can be made by etching after orbefore the crystallization.

Implementation Examples

[0048] On the surface of non-alkali glass sheet, manufactured by CorningGlass Works, with the outside dimension of 400×500 mm, a thickness of0.7 mm and a strain point of 650° C., a layer of silicon oxide (SiO2)with thickness of 200 nm was deposited by the method of plasma CVD, asthe first control layer for heat-conduction and crystallization. Then, alayer of amorphous silicon (a-Si:H) with thickness of 60 nm wasdeposited without being exposed to the atmosphere. Next, this layer ofamorphous semiconductor was annealed, and after dehydrogenating it, thesurface area of the layer is formed as an oxygen-implanted area byimplantation of oxygen ion. The implantation of oxygen ion was carriedout with the accelerated voltage of 3 keV, and the dose amount of1.5E17/cm². Under these conditions, the position of the maximum oxygendensity was at the depth of about 10 nm corresponding to ion projectionrange (Rp), and the maximum density of oxygen was 1E23/cm³.

[0049] As stated in the foregoing, the implanting amount of oxygen (doseamount) or the implanting depth of oxygen (Rp) is determined by thethickness of insulating oxidized layer and its position to be formed. Inthe present example, the above implantation conditions were determinedfrom the viewpoint for forming an insulation oxidized layer of about 30nm thicknesses on the surface of a layer of single-crystallinesemiconductor.

[0050] Sequentially, pulsated laser beam were irradiated to thesubstrate sheet from its upper-side, whereby the amorphous silicon layerwas crystallized and the oxygen implanted layer was converted to anoxidized layer. The irradiation was carried out in a mode that one unitof laser has a square size irradiation face of 5 mm×5 mm in which250,000 maximum intensity points and minimum intensity points arearranged with intervals of 10 μm in a square lattice form, by using aphase shift mark for distributing the irradiation strength. In thisembodiment, the melting threshold value (Emth) was about 0.6 J/cm², themaximum energy strength of laser beam (Emax) being 1.9 J/cm² and minimumstrength value (Emin) being 0.1 J/cm².

[0051] By the irradiation process, the layer of amorphous silicon of 60nm thickness was converted to a layer comprising a crystalline siliconlayer of about 50 nm thickness and an oxidized layer of about 30 nmthickness. Implanted oxygen ion of dose amount of 1.5 E17/cm² is reactedwith silicon atoms equivalent to silicon layer of 20 nm thickness, andformed a silicon oxide layer of about 30 nm thickness.

[0052] The irradiation by excimer laser in this process was carried outover the whole surface of the sheet by shifting the irradiation facestepwise at the intervals of 5 mm. After finishing the irradiation, aSecco-etching treatment was made. As the result of observation by ascanning-type electronic microscope, it was found that a substrate forthin film semiconductor device, in which one million numbers ofsingle-crystalline grains of average size of 4.5 μm were arrangedregularly in the lattice form in every square areas of 5×5 mm.

[0053] Next, a molybdenum/tungsten alloy layer is deposited by asputtering process, and gate electrodes were formed by patterning thelayer to predetermined shapes while adjusting its position so as tocorresponding to the position of single-crystalline grains. Then, byimplanting phosphorous ions using the gate electrodes as the mask,source electrodes and drain electrodes were formed. Then, layers ofsilicon oxide were deposited by a plasma CVD process for forming aninsulator. After perforating contact holes in insulator positionscorresponding to source areas and drain areas, aluminum layers aredeposited and patterned thereon, thereby completed a thin filmtransistor (TFT). This device acted with N-channel operation, showingthe threshold voltage (Vth) of 1.2 V and the mobility of 496 cm2/V·sec.When thin film transistors were formed on a thin film semiconductorsubstrate sheet of 400 mm×500 mm size, threshold values thereof were 1.2V±0.08V and the mobility were 496±56 cm2/V·sec. In a BTS estimation of10,000 seconds, shifting values were only 0.05V.

[0054] The following is another example, in which the insulatingoxidized film layer is embedded into the film of single-crystallinesilicon grains.

[0055] On the surface of base layer of non alkali glass manufactured byComing Glass Works, having the dimension of 400 mm×500 mm, a thicknessof 0.7 mm, and a strain point of 650° C., an oxidized silicon (SiO²)film of 200 nm thickness is formed by plasma CVD method as the firstcontrol layer for heat conduction and crystallization. Then, a layer ofamorphous silicon (a-Si:H) of 110 nm thickness is formed withoutexposing to the atmosphere.

[0056] The layer of amorphous silicon oxide was annealed anddehydrogenated. Then, oxygen ions were implanted into the intermediateregion in the thickness direction in order to form an oxygen implantedarea. The implantation of oxygen ions was carried out with theaccelerated voltage of 20 keV and the dose amount of 1.5E17/cm². Underthese conditions, the maximum oxygen density point of 3E22/cm³ waspositioned in the depth of about 50 nm corresponding to the projectionrange of oxygen ions. The above conditions for formation of theimplantation area were determined for the purpose of forming aninsulating oxidized layer of about 30 nm thickness in the depth of about60 nm (central depth) from the surface of the single-crystalline grainslayer.

[0057] Then, the pulsated excimer laser beam was used to irradiate thelayer from the upper direction, whereby the amorphous silicon layer wascrystallized and the oxygen implanted areas were converted to oxidizedareas. The irradiation by laser was carried out with the irradiationintensity distribution by using a phase shifting mask so that a total of250,000 maximum intensity points and minimum intensity points wereformed arranged within a square lattice formed at intervals of 10 μm inone unit of irradiated face of 5 mm×5 mm. In this example, meltingthreshold value (Eth) was about 0.8 J/cm², the maximum value of energyintensity (Emax) was 2.3 J/cm² and the minimum intensity value was 0.1J/cm². By the above irradiation, the amorphous silicon layer of 110 nmthickness was changed to three layers, namely, the secondsingle-crystalline silicon grains layer at the top area with 45 nmthickness, the oxidized silicon layer of about 30 nm at the intermediatearea and the first single-crystalline grains layer of about 50 nm at thebottom area.

[0058] Next, the second single crystalline silicon layer 55 was formedas a gate electrode by patterning, and a source area and drain area wereformed by implanting phosphorous ions using the gate electrode as amask. Following this, a silicon oxide layer was deposited as aninsulator using a plasma CVD deposition method, and contact holes wereformed in the insulator at the position above the source and drainareas. The thin film transistor (TFT) is then completed by depositingaluminum and patterning it. This process results in a N channel devicewith a threshold voltage value (Vth) of 1.0V and an electron mobility of475 cm²/V·sec.

[0059] The threshold values of 20 numbers of thin film transistors, eachof which was formed on the thin film semiconductor base layer of 400nm×500 nm, were 1.0±0.08 V, and the mobilities thereof were 475±50cm²/V·sec. Furthermore, by the BTS estimation of 10,000 seconds, shiftamount of Vth was only 0.05 V.

[0060] A number of embodiments of the present invention have beendescribed. Nevertheless, it will be understood that variousmodifications may be made without departing from the spirit and scope ofthe invention. For example, while in the above examples the oxidizelayer is formed by implanting oxygen ion, the layer can be formed byfurther introducing other ions such as nitrogen ion. Accordingly, otherembodiments are within the scope of the following claims.

What is claimed is:
 1. A substrate sheet for thin film semiconductordevices comprising: a base layer of insulation materials; and a thinfilm semiconductor layer formed on the base layer, said thin filmsemiconductor layer comprising a layer of crystalline semiconductorgrains formed by crystallization or recrystallization of a layer ofnon-single-crystalline semiconductor and an oxidized layer formed byoxidization of said layer of non-single crystalline semiconductor.
 2. Asubstrate sheet for thin film semiconductor devices of claim 1, whereina plurality of single-crystalline grains are arranged in a regulatedmode in the layer of crystalline semiconductor.
 3. A substrate sheet forthin film semiconductor devices of claim 1, wherein the thin oxidizedlayer formed by oxidation of non-single-crystalline semiconductor layeris formed as a top layer of the thin film semiconductor layer.
 4. Asubstrate sheet for thin film semiconductor devices of claim 1, whereinthe thin oxidized layer formed by oxidation of non-single-crystallinesemiconductor layer is formed as an intermediate layer in the thin filmsemiconductor layer.
 5. A substrate sheet for thin film semiconductordevices of claim 1, wherein the grain size of single-crystallinesemiconductor is at least 2 μm.
 6. A substrate sheet for thin filmsemiconductor devices of claim 1, wherein a control layer forheat-conduction or crystallization is formed between the base layer ofinsulation material and the thin film semiconductor layer.
 7. Asubstrate sheet for thin film semiconductor devices of claim 2, whereinthe thin oxidized layer formed by oxidation of non-single-crystallinesemiconductor layer is formed as a top layer of the thin filmsemiconductor layer.
 8. A substrate sheet for thin film semiconductordevices of claim 2, wherein the thin oxidized layer formed by oxidationof non-single-crystalline semiconductor layer is formed as anintermediate layer in the thin film semiconductor layer.
 9. A substratesheet for thin film semiconductor devices of claim 2, wherein the grainsize of single-crystalline semiconductor is at least 2 μm.
 10. Asubstrate sheet for thin film semiconductor devices of claim 2, whereina control layer for heat-conduction or crystallization is formed betweenthe base layer of insulation material and the thin film semiconductorlayer.
 11. A thin film semiconductor device comprising; a base layer ofinsulation materials; and a thin film semiconductor layer formed on thebase layer, said thin film semiconductor layer comprising a layer ofsemiconductor crystal grains formed by crystallization orrecrystallization of non-single-crystalline semiconductor layer and anoxidized layer formed by oxidization of said non-single-crystallinesemiconductor layer and, the area of said thin oxidized layerconstituting a insulator of gate electrode.
 12. A thin filmsemiconductor device of claim 11, wherein semiconductor crystal grainsare arranged in a regulated mode in the layer of single-crystallinesemiconductor.
 13. A thin film semiconductor device of claim 11, whereinthe grain size of single-crystalline semiconductor grains formed in thelayer of single-crystalline semiconductor grains is at least 2 μm.
 14. Athin film semiconductor device of claim 11, wherein a control layer forheat conduction and recrystallization is formed between the base layerof insulation materials and the thin film semiconductor layer.
 15. Amethod for producing a substrate sheet for thin film semiconductordevices comprising steps of; (a) depositing a layer ofnon-single-crystalline semiconductor on a base layer of insulationmaterials, (b) forming oxygen implanted areas in the layer ofnon-single-crystalline by implanting oxygen ion into the layer, and (c)irradiating the layer of non-single-crystalline semiconductor withenergy beam, thereby changing the layer of non-single-crystallinesemiconductor so that the oxygen implanted areas are converted toinsulating oxidized films and other areas are converted to films ofsemiconductor crystal grains.
 16. A method for producing a substrate forthin film semiconductor devices of claim 15, wherein the irradiation ofenergy beam is carried out so that the area to which the irradiationintensity of maximum value is given and the area to which theirradiation intensity of minimum value is given are arranged in aregulated mode and the transition of irradiation intensity between theabove two areas are successive.
 17. A method for producing a substratefor thin film semiconductor devices of claim 15, wherein the minimumvalue of irradiation intensity the intensity which does not cause themelt of the non-single-crystalline semiconductor.
 18. A method forproducing a substrate sheet for thin film semiconductor devices of claim15, wherein the oxygen implanted areas are formed in the top layerportion of non-single-crystalline semiconductor layer.
 19. A method forproducing a substrate sheet for thin film semiconductor devices of claim15, wherein the oxygen implanted areas are formed in the intermediatelayer portion of non-single-crystalline semiconductor layer.
 20. Amethod for producing a thin film semiconductor devices comprising stepsof; (a) depositing a layer of non-single-crystalline semiconductor on abase layer of insulation materials, (b) forming oxygen implanted areasin the layer of non-single-crystalline semiconductor by implantingoxygen ion into the layer, (c) irradiating the layer ofnon-single-crystalline with energy beam, thereby changing the layer ofnon-single -crystalline semiconductor so that the oxygen implanted areasare converted to insulating oxidized films and other areas are convertedto films of semiconductor crystal grains, (d) forming a gate electrodeby patterning the layer of semiconductor crystal grains and using theinsulating oxidized films as a gate insulator and, (e) completing anelectric circuit unit by forming a source electrode and a drainelectrode in the layer of semiconductor crystal grains.
 21. A method forproducing a thin film semiconductor device of claim 20, wherein theirradiation of energy beam is carried out so that the area to which theirradiation intensity of maximum value in given and the area to whichthe irradiation intensity of are minimum value is given are arranged ina regulated mode and the transition of irradiation intensity between theabove two areas are successive.
 22. A method for producing a thin filmsemiconductor device of claim 20, wherein the insulating oxidized layeris formed at the top layer portion of thin film semiconductor layer and,the resulted insulating oxidized layer is used as the gate insulator andthe source electrode and the drain electrode are formed in the lowerlayer of crystal grains.
 23. A method for producing a thin filmsemiconductor device of claim 20, wherein the insulating oxidized layeris formed in the intermediate layer portion of thin film semiconductorlayer and, the resulted insulating oxidized layer is used as the gateinsulation layer and the source electrode and the drain electrode areformed in the lowest layer of semiconductor grains.